Ground stock generally has various grain sizes at the end of the grinding process and requires classification or separation. The mill and the separator are consequently integrated, in general, into one functional unit. The starting product is charged into the mill, and at the end of the grinding process, the ground stock, freed from coarse particles, reaches the separator, where the coarser and finer components of the ground stock (which have a heavier and lighter weight, respectively), are separated from one another.
The grinder and the separator are subject to different working conditions, which can be readily taken into account or can be readily created if the grinder and the separator are mutually independent units.
If the separator is an air separator, the essential criterion for the separator is the distribution number (the cut point) d.sub.T, which is described by the following equation. ##EQU1## Here, d.sub.T is the distribution number (the cut point) or the particle size,
.rho. is the density in kg/m.sup.3, PA1 V.sub.r is the radial velocity in the separator wheel in m/sec, PA1 V.sub.u is the circumferential velocity in the separator wheel in m/sec, PA1 r is the radius in m, and PA1 n is the dynamic viscosity in Ns/m.sup.2.
It follows from this that the distribution number in the separator depends mainly on two velocity components, which in turn means that the speed of the separator wheel must be variable if the working range of the separator is not to be so small that the separator operation would be uneconomical.
This analogously applies to the mill as well. In the case of a beater mill with beaters on the circumference of a rotating disk, the speed of this disk must be variable if the beater mill is to be operated economically.
The lack of a fixed relationship between the speeds of the mill and the separator is an important aspect for the design of a beater mill and an air separator. The speeds of the separator and the mill must be able to be determined and varied independently from one another. This does not represent any special problem if the separator and the mill are mutually independent devices with mutually independent drives.
Problems arise from the above-described situation if the mill and the separator are to be connected to one another not only functionally, but also structurally in order to reduce the cost of construction, or in order to quite simply keep the space required for installation small. It is logical in such a case to provide a common drive motor for the two devices and to provide a torque split transmission between the motor and the mill, on the one hand, and the separator, on the other hand, and to now provide a speed control between the torque split transmission and the mill, on the one hand, and the torque split transmission and the separator, on the other hand. However, this makes the entire machine with the mill and the separator so expensive that other advantages, which are expected from the structural integration, would be offset. Separate drives for the separator and the mill with speed controls of their own have therefore been maintained to date in practice even in the case of the structural integration of the mill and the separator.